WO2019188094A1 - Ferritic stainless steel sheet and method for producing same - Google Patents

Abstract

This ferritic stainless steel sheet contains 11.0-30.0% of Cr, 0.001-0.030% of C, 0.01-2.00% of Si, 0.01-2.00% of Mn, 0.003-0.100% of P, 0.0100% or less of S, 0.030% or less of N, 0-0.0025% of B, 0-0.50% of Sn, 0-1.00% of Ni, 0-1.00% of Cu, 0-2.00% of Mo, 0-1.00% of W, 0-1.00% of Al, 0-0.50% of Co, 0-0.50% of V, 0-0.50% of Zr, 0-0.0050% of Ca, 0-0.0050% of Mg, 0-0.10% of Y, 0-0.10% of Hf, 0-0.10% of REM, 0-0.50% of Sb, and 0.40% or less of Ti and/or 0.50% or less of Nb, with the balance being made up of Fe and impurities. With respect to this ferritic stainless steel sheet, the amount of P that is present in the form of phosphides is 0.003% by mass or more; and the crystal grain size number thereof as determined in accordance with JIS G 0551 is 9.0 or greater.

Description

Ferritic stainless steel sheet and manufacturing method thereof

The present invention relates to a ferritic stainless steel sheet and a method for producing the same, and more particularly, to a ferritic stainless steel sheet having excellent formability during forming and resistance to roughened workability, and a method for producing the same.
This application claims priority based on Japanese Patent Application No. 2018-069775 filed in Japan on March 30, 2018, the contents of which are incorporated herein by reference.

SUS304 (18Cr-8Ni), which is a representative steel type of austenitic stainless steel, is widely used in home appliances, kitchen products, building materials and the like because of its excellent corrosion resistance, workability, and beauty. However, since SUS304 has a large amount of expensive Ni with high price fluctuation, it is said that the price of the steel sheet is high. On the other hand, ferritic stainless steel does not contain Ni or has a very low content, and therefore demand is increasing as a material with excellent cost performance. However, when ferritic stainless steel is used as a molding application, the problems are the molding limit and the deterioration of the rough surface due to the formation of surface irregularities after molding.

First, when comparing the forming limits, the austenitic stainless steel is excellent in the overhanging property, but the overhanging property of the ferritic stainless steel is low and the shape cannot be changed greatly. However, since deep drawability can be controlled by adjusting the crystal orientation (texture), when a ferritic stainless steel is used as a forming application, a forming method mainly using deep drawing is often used.

Next, surface characteristics after the molding process, particularly rough surface (surface irregularities after molding) will be described. Here, “surface irregularities” refers to fine irregularities (skin roughness) that occur on the surface of a steel sheet after processing or forming, and these fine irregularities correspond to crystal grains. Surface irregularities are also noticeable.
In the case of austenitic stainless steel, a steel sheet having a crystal grain size number of about 10 is manufactured because it is excellent in work hardening characteristics and relatively easy to produce a fine grain structure. For this reason, the surface irregularities (skin roughness) after molding are small and hardly cause a problem. On the other hand, the grain size of ferritic stainless steel is about 9 for SUS430 and about 7 for SUS430LX, which is smaller than that of austenitic stainless steel. Here, a small particle size number indicates that the crystal grain size is large.
The reason why ferritic stainless steel is likely to become coarse grains is that ferritic stainless steel tends to have a large recrystallized grain size and, like SUS430LX, reduces C and N to improve workability and formability. This is because the high-purity ferritic stainless steel that has been improved easily grows. Further, in ferritic stainless steel, even when a product plate having a fine crystal grain size is manufactured by increasing the number of cold rolling, rough skin may be generated, and the cause is not necessarily clear.

When relatively strict formability is required, such as a housing or container of home appliances, a high purity ferritic stainless steel such as SUS430LX is often used as the ferritic stainless steel. In addition, in order to ensure the strength after forming, the thickness of the stainless steel plate used is 0.6 mm or more in most cases. However, as described above, ferritic stainless steel has a large crystal grain size, so The surface roughness of the surface is large, and surface irregularities are usually removed by polishing.

From the background described above, a technique for reducing the rough skin of high purity ferritic stainless steel has been disclosed.
Patent Document 1 discloses a ferritic stainless steel excellent in formability with less roughened working surface by controlling the size and crystal grain size of precipitated particles using high purity ferritic stainless steel and a method for producing the same. Yes. However, in Patent Document 1, although a steel sheet having a small crystal grain size is obtained, the deep drawability at the time of molding is not sufficient, and the rough surface after molding tends to occur despite the small crystal grain size. There was a problem.

In Patent Document 2, in ferritic stainless steel containing Ti and Nb, hot rolling is performed at a low temperature, and a fine grain is obtained by taking a high cold rolling rate, which is excellent in rough skin resistance at the time of molding. A technique for producing stainless steel is disclosed. According to such a technique, the stainless steel of Patent Document 2 has a crystal grain size number of 9.5 and a fine-grained structure, but the skin roughness after cup drawing is not always sufficient.

Patent Document 3 discloses a ferritic stainless steel excellent in deep drawability, ridging properties and skin roughness resistance by controlling the crystal grain size before final cold rolling of steel having a component composition containing Nb and / or Ti. Is disclosed. However, in Patent Document 3, the crystal grain size of the final product is 15 μm (crystal grain size number: 9.1), and the rough skin property is insufficient.

As described above, when considering the forming process of ferritic stainless steel, it is very difficult to form into a predetermined shape and to satisfy the surface characteristics after forming. For this reason, when using ferritic stainless steel as a forming application, it is necessary to perform a polishing step in order to remove surface irregularities generated after forming. However, this polishing process takes time for polishing and increases the manufacturing cost. Further, there is a problem that a lot of dust generated by polishing is generated.

Japanese Patent No. 4749888 JP 7-292417 A Japanese Patent No. 3788311

The present invention has been made in view of the above problems, and provides a ferritic stainless steel sheet excellent in forming workability and resistance to roughening after forming, and a method for producing the same.

Crystal grain size and amount of strain are known as factors affecting the rough processing of ferritic stainless steel. However, as described above, rough machining may occur even if the crystal grain size and strain are increased by controlling the cold rolling conditions and the like, and in recent years, steels that can more stably suppress the occurrence of rough machining have been desired. It was.
Therefore, the present inventors investigated the relationship between the roughened working surface and the metal structure in ferritic stainless steel. For the first time, it has been found that not only the crystal grain size and the strain amount that have been conventionally known, but also the precipitation amount of precipitates in steel affects the roughening of the work surface. In addition, in order to control the amount of precipitation within an appropriate range, it was necessary to control the heat treatment temperature before and after cold rolling, and it was also clarified that rapid heating was necessary in the heat treatment after cold rolling.

The summary of one embodiment of the present invention is as follows.
[1] By mass%
Cr: 11.0% or more and 30.0% or less,
C: 0.001% to 0.030%,
Si: 0.01% or more and 2.00% or less,
Mn: 0.01% or more and 2.00% or less,
P: 0.003% to 0.100%,
S: 0.0100% or less,
N: 0.030% or less,
B: 0% or more and 0.0025% or less,
Sn: 0% to 0.50%,
Ni: 0% or more and 1.00% or less,
Cu: 0% or more and 1.00% or less,
Mo: 0% or more and 2.00% or less,
W: 0% to 1.00%,
Al: 0% or more and 1.00% or less,
Co: 0% to 0.50%,
V: 0% or more and 0.50% or less,
Zr: 0% or more and 0.50% or less,
Ca: 0% or more and 0.0050% or less,
Mg: 0% or more and 0.0050% or less,
Y: 0% or more and 0.10% or less,
Hf: 0% or more and 0.10% or less,
REM: 0% or more and 0.10% or less,
Sb: 0% or more and 0.50% or less,
Ti: 0.40% or less, Nb: 0.50% or less, including either one or both, the balance consists of Fe and impurities,
The amount of P present as a phosphide is 0.003% by mass or more,
A ferritic stainless steel sheet having a grain size number measured by JIS G 0551 of 9.0 or more.
[2] In mass%,
B: 0.0001% to 0.0025%,
Sn: 0.005% or more and 0.50% or less,
Ni: 0.05% or more and 1.00% or less,
Cu: 0.05% or more and 1.00% or less,
Mo: 0.05% or more and 2.00% or less,
W: 0.05% or more and 1.00% or less,
Al: 0.05% or more and 1.00% or less,
Co: 0.05% or more and 0.50% or less,
V: 0.05% or more and 0.50% or less,
Zr: 0.05% or more and 0.50% or less,
Ca: 0.0001% to 0.0050%,
Mg: 0.0001% or more and 0.0050% or less,
Y: 0.001% or more and 0.10% or less,
Hf: 0.001% or more and 0.10% or less,
REM: 0.001% or more and 0.10% or less,
Sb: 0.005% or more and 0.50% or less of 1 type or 2 types or more are contained, The ferritic stainless steel plate as described in said [1] characterized by the above-mentioned.

[3] A hot rolling process in which the steel having the component described in [1] or [2] is hot-rolled, and heat that is heat-treated at a temperature of 850 ° C. or higher and 900 ° C. or lower after the hot rolling process. A rolled sheet annealing process, a cold rolling process for rolling at a rolling rate of 75% to 90% after the hot rolled sheet annealing process, and a cold rolled sheet annealing process performed subsequent to the cold rolling process. In the cold rolled sheet annealing step, the average rate of temperature increase in the temperature range of 400 ° C. to 800 ° C. in the temperature increasing process is 80 ° C./s or more, and the maximum temperature reached is 880 ° C. or more and 980 ° C. or less. The above [1] or characterized in that the cooling is started within 5 seconds after reaching the maximum temperature and the average cooling rate in the temperature range from the maximum temperature to 700 ° C. is 50 ° C./s or more. [2] Ferritic stainless steel according to Manufacturing method of the scan steel plate.
[4] A hot rolling step in which the steel having the component according to [1] or [2] is hot-rolled, and a heat treatment is performed at a temperature of 850 ° C. or higher and 900 ° C. or lower after the hot rolling step. A hot-rolled sheet annealing step in which the amount of P existing as a phosphide is 0.003% by mass or more, and a cold rolling step in which the rolling rate is rolled at 75% to 90% after the hot-rolled sheet annealing step; A cold-rolled sheet annealing step performed subsequent to the cold-rolling step, and in the cold-rolled sheet annealing step, an average temperature increase rate in a temperature range of 400 ° C. to 800 ° C. is 80 ° C./s. The maximum temperature of the plate temperature is not less than 880 ° C. and not more than 980 ° C., cooling is started within 5 seconds after reaching the maximum temperature, and the average cooling rate in the temperature range from the maximum temperature to 700 ° C. is 50 Cooling at more than ℃ / s The symptom, method for manufacturing the ferritic stainless steel sheet according to [1] or [2].

According to one aspect of the present invention, it is possible to provide a ferritic stainless steel sheet that is excellent in forming processability and resistance to roughening of the processed skin after forming process.

It is a TEM observation result (TEM photograph) of the recrystallized structure of the ferritic stainless steel plate which concerns on this embodiment. It is a figure which shows the relationship between the crystal grain size number which concerns on a present Example, and the precipitation amount (Pp) of P.

Hereinafter, each requirement of the ferritic stainless steel sheet according to an embodiment of the present invention will be described in detail. In addition, "%" display of the content of each element means "mass%".

The reason for limiting the component (I) will be described below.

Cr is an element that improves the corrosion resistance, which is a basic characteristic of stainless steel. If it is less than 11.0%, sufficient corrosion resistance cannot be obtained, so the lower limit is made 11.0% or more. On the other hand, if an excessive amount of Cr is contained, the formation of an intermetallic compound corresponding to the σ phase (Fe—Cr intermetallic compound) is promoted to promote cracking during production, so the upper limit is 30.0% or less. To do. From 14.0% or more and 25.0% or less are desirable from the viewpoint of stable manufacturability (yield, rolling mill, etc.). More preferably, it is 16.0% or more and 20.0% or less.

C is an element that lowers the formability that is important in the present embodiment, so it is preferable that C be less, and the upper limit is 0.030% or less. However, excessive reduction leads to an increase in refining costs, so the lower limit is made 0.001% or more. In consideration of both the refining cost and the moldability, 0.002% or more and 0.020% or less are preferable.

Si is an element that improves the oxidation resistance. However, if an excessive amount of Si is contained, the moldability is lowered, so the upper limit is made 2.00% or less. From the viewpoint of formability, the Si content is preferably low, but excessive reduction leads to an increase in raw material cost, so the lower limit is made 0.01% or more. From the viewpoint of manufacturability, the desirable range is 0.05% or more and 1.00% or less, and more desirably 0.05% or more and 0.30% or less.

Since Mn, like Si, contains a large amount of Mn, the formability is lowered, so the upper limit is made 2.00% or less. Although it is preferable that the amount of Mn is low from the viewpoint of moldability, excessive reduction causes an increase in raw material cost, so the lower limit is made 0.01% or more. From the viewpoint of manufacturability, the desirable range is 0.05% or more and 1.00% or less, and more desirably 0.05% or more and 0.30% or less.

P is an important element that contributes to the improvement of the rough surface resistance to processing by precipitating as a phosphide in the steel sheet of the present embodiment. In order to secure the amount of precipitation of phosphide and improve the resistance to rough processing, the P amount is set to 0.003% or more. However, since P is an element that lowers moldability, the upper limit is made 0.100% or less. In addition, excessive reduction of the amount of P brings about an increase in the raw material cost, and when considering both formability and rough processing resistance, a preferable range is 0.010% or more and 0.050% or less, and further desirably. Is 0.020% or more and 0.040% or less.

S is an impurity element and is preferably lower because it promotes cracking during production. The upper limit is 0.0100% or less. The lower the amount of S, the better, and 0.0030% or less is desirable. On the other hand, an excessive decrease leads to an increase in refining costs, so the lower limit is preferably 0.0003% or more. From the viewpoint of manufacturability and cost, the preferred range is 0.0004% or more and 0.0020% or less.

N is an element that lowers the formability like C, and the upper limit is 0.030% or less. However, excessive reduction leads to an increase in refining costs, so the lower limit is preferably made 0.002% or more. From the viewpoint of moldability and manufacturability, the preferred range is 0.005% or more and 0.015% or less.

One or both of Ti and Nb are contained as follows.
Ti combines with C and N, fixes C and N as precipitates such as TiC and TiN, and improves r value and product elongation through high purity. In order to obtain these effects, when Ti is contained, the lower limit is preferably set to 0.03% or more. On the other hand, if it is excessively contained, the alloy cost is increased and the manufacturability is lowered with the increase of the recrystallization temperature, so the upper limit is made 0.40% or less. From the viewpoint of moldability and manufacturability, the preferred range is 0.05% or more and 0.30% or less. Furthermore, the suitable range which utilizes the said effect of Ti actively is 0.10% or more and 0.20% or less.

Nb is also a stabilizing element that fixes C and N in the same manner as Ti, and improves r value and product elongation through high purity of steel by this action. In order to obtain these effects, when Nb is contained, the lower limit is preferably set to 0.03% or more. On the other hand, if it is excessively contained, it leads to a decrease in manufacturability accompanying an increase in alloy costs and a recrystallization temperature, so the upper limit is made 0.50% or less. From the viewpoint of alloy cost and manufacturability, the preferred range is 0.03% or more and 0.30% or less. Furthermore, the suitable range which utilizes the said effect of Nb actively is 0.04% or more and 0.15% or less. More preferably, it is 0.06 to 0.10%.

The ferritic stainless steel sheet of the present embodiment is composed of Fe and impurities other than the elements described above (remainder). In the present embodiment, in addition to the basic composition described above, one or more of the following element groups or Two or more kinds may be selectively contained. That is, the lower limit of the content of B, Sn, Ni, Cu, Mo, W, Al, Co, V, Zr, Ca, Mg, Y, Hf, REM, and Sb is 0% or more.
The “impurities” in the present embodiment are components that are mixed due to various factors in the manufacturing process including raw materials such as ores and scraps when industrially manufacturing steel, and are inevitably mixed. Including ingredients.

B is an element that improves secondary workability. Since 0.0001% or more is necessary to exert the effect, this is the lower limit. On the other hand, if excessively contained, the productivity, particularly castability, is deteriorated, so the upper limit is made 0.0025% or less. A preferable range is 0.0003% or more and 0.0012% or less.

Since Sn is an element having an effect of improving the corrosion resistance, it may be contained according to the corrosive environment at room temperature. Since the effect is exhibited at 0.005% or more, this is the lower limit. On the other hand, if contained in a large amount, the productivity is deteriorated, so the upper limit is made 0.50% or less. Considering manufacturability, the preferred range is 0.02% or less and 0.10% or less.

Ni, Cu, Mo, Al, W, Co, V, and Zr are effective elements for enhancing the corrosion resistance or oxidation resistance, and may be contained as necessary. The effect is manifested by setting each content of Ni, Cu, Mo, Al, W, Co, V, and Zr to 0.05% or more. However, when it contains excessively, not only the moldability will fall, but it will lead to an increase in alloy cost and to obstruct manufacturability. Therefore, the upper limit of Ni, Cu, Al, and W is made 1.00% or less. The upper limit of Ni, Cu, Al, and W is preferably 0.50% or less. Since Mo causes a decrease in manufacturability, the upper limit is made 2.00% or less. The upper limit of Mo is preferably 1.00% or less. The upper limit of Co, V, and Zr is set to 0.50% or less in consideration of the manifestation of the effect of improving the corrosion resistance or oxidation resistance. The lower limit of the more preferable content of any element of Ni, Cu, Mo, Al, W, Co, V, and Zr is 0.10% or more.

Ca and Mg are elements that improve hot workability and secondary workability, and may be contained as necessary. However, if it is excessively contained, it will lead to inhibition of manufacturability, so the upper limit of Ca and Mg is made 0.0050% or less. Preferred lower limits are both 0.0001% or more. In consideration of manufacturability and hot workability, a preferable range for both Ca and Mg is 0.0002% or more and 0.0010% or less.

Y, Hf, and REM are effective elements for improving hot workability, cleanliness of steel, and improving oxidation resistance, and may be contained as necessary. When it contains, an upper limit shall be 0.10% or less, respectively. The preferable lower limit is 0.001% or more for Y, Hf, and REM. Here, “REM” in the present embodiment is composed of one or more elements selected from an element group (lanthanoid) belonging to atomic numbers 57 to 71, such as La, Ce, Pr, and Nd. It is. The “REM” content in the present embodiment is the total amount of lanthanoids.

Sb is an element having an effect of improving the corrosion resistance like Sn, and may be contained if necessary. However, if contained in a large amount, the productivity is deteriorated, so the upper limit is made 0.50% or less. On the other hand, since the effect of improving the corrosion resistance is exhibited at 0.005% or more, this is the lower limit.

The ferritic stainless steel sheet of the present embodiment is composed of Fe and impurities (including inevitable impurities) other than the elements described above, but the effects of the present embodiment are not impaired in addition to the elements described above. It can be contained in a range. In the present embodiment, for example, Bi, Pb, Se, H, Ta and the like may be contained, but in that case, it is preferable to reduce as much as possible. On the other hand, the content ratio of these elements is controlled to the extent that solves the problem of the present embodiment, and if necessary, Bi ≦ 100 ppm, Pb ≦ 100 ppm, Se ≦ 100 ppm, H ≦ 100 ppm, Ta ≦ 500 ppm. You may contain 1 or more types.

(II) Next, the metal structure will be described.
The ferritic stainless steel plate of the present embodiment is composed of a ferrite single phase structure having a crystal grain size number of 9.0 or more.
The grain size number is 9.0 or more. The roughening of the processed skin after molding is less likely to occur as the grain size number is larger, that is, as the grain size of the ferrite crystal grains is smaller. In order to further suppress rough skin, it is preferably over 9.5, more preferably over 10.0. However, if the grain size of the crystal grains becomes excessively small, the strength may increase and the press moldability may decrease. For this reason, the crystal grain size number is preferably 12 or less.

The crystal grain size number can be obtained by the line segment method of JIS G 0551 (2013). “Granularity number: 9” corresponds to an average line segment length of 14.1 μm per crystal grain traversing the crystal grain, and “grain size number: 10” is one crystal traversing the crystal grain. This corresponds to an average line segment length per grain of 10.0 μm. In the measurement of the crystal grain size, the number of crystal grains crossing per sample is set to 500 or more from an optical micrograph of the cross section of the test piece. The etchant is preferably aqua regia or reverse aqua regia, but other solutions may be used as long as the crystal grain boundaries can be determined. Further, depending on the orientation relationship between adjacent crystal grains, the grain boundary may not be seen clearly, so that it is preferable to etch deeply. In measuring grain boundaries, twin grain boundaries are not measured.

Further, the metal structure of the ferritic stainless steel plate of the present embodiment is composed of a ferrite single phase structure, and P precipitates (phosphides) described later are generated. This means that an austenite phase and a martensite structure are not included. This is because when an austenite phase or a martensite structure is included, it is relatively easy to reduce the crystal grain size. Furthermore, the austenite phase exhibits high formability due to the TRIP effect. However, in addition to an increase in raw material cost, a yield reduction such as an ear crack is likely to occur at the time of manufacture, so the metal structure is a ferrite single phase structure. In addition, precipitates such as carbonitrides may exist in the steel other than phosphides, but these do not take into account the effect of this embodiment, so these are not considered, the above is the main phase Describes the organization.

(III) Next, the precipitation amount of P will be described.
Usually, P in a ferritic stainless steel sheet reduces the formability (r value and product elongation), so it is considered that its content should be reduced. However, as a result of the study by the present inventors, it has been found for the first time that the amount of precipitation of phosphide in steel affects the roughened working surface. From this, in this embodiment, it is clear that, in addition to controlling the crystal grain size, by controlling the amount of P present as a phosphide, that is, the precipitation amount Pp of P, it is possible to stably suppress rough processing skin. And the amount of precipitation P of P is defined.

Thus, since the phosphide in steel greatly contributes to the suppression of roughening of the processed skin, it is necessary to ensure the amount of precipitation of P. Therefore, in this embodiment, the amount of P existing as a phosphide (P precipitation amount Pp) is set to 0.003% by mass or more. Desirably, it is 0.004 mass% or more, More preferably, it is 0.005 mass% or more. The upper limit of the P precipitation amount Pp is not particularly limited, but since the upper limit of the P content of the steel sheet is 0.100% or less, the upper limit of the P precipitation amount Pp may be 0.100% or less. The phosphide referred to in the present embodiment includes, for example, Fe phosphide, Mn phosphide, Ti phosphide, Nb phosphide, Al phosphide, etc., but the type and composition are not particularly limited. That is, in this embodiment, it is important that the amount of P existing as a phosphide (P precipitation amount Pp) is within the above range regardless of the specific composition and form of the phosphide.

Although the details of the method for controlling the precipitation amount Pp of P within the above range will be described later, the processing temperature of the heat treatment (hot rolled sheet annealing and finish annealing) performed before and after the cold rolling process is controlled, and cold rolling is performed. It can be controlled by rapidly performing the heating process in the subsequent heat treatment.

The cause of the precipitated phosphide contributing to the roughening of the processed skin is under intensive investigation, but at the present time, it is considered as follows.
In general, since precipitates are likely to precipitate on the grain boundaries, it is considered that many of the phosphides precipitated by hot-rolled sheet annealing are also precipitated on the grain boundaries. Thereafter, it is considered that the phosphide precipitated on the grain boundaries is aligned in parallel with the rolling direction as the metal structure is crushed by cold rolling and extends in the rolling direction. From this state, if recrystallization is performed by applying rapid annealing, holding for a short time, and rapid cooling, a recrystallized structure of the metal structure can be obtained with almost no change in the precipitation state of the phosphide. That is, by performing rapid annealing, holding for a short time, and rapid cooling for the finish annealing, a recrystallized structure is obtained in which phosphides are maintained in a state parallel to the rolling direction.
In fact, the present inventors found that the phosphide in the crystal grains of the recrystallized structure is parallel to the rolling direction in the thin film TEM observation of the product plate manufactured by such a manufacturing method (within the manufacturing method range of the present embodiment described later). You can see how they are lined up. FIG. 1 shows a TEM observation result of a recrystallized structure in a steel sheet manufactured under conditions that satisfy the present embodiment described later. As is clear from FIG. 1, it can be confirmed that the P compound is precipitated along the rolling direction in the crystal grains of the recrystallized structure. Note that whether or not the precipitates precipitated in the crystal grains are P compounds was identified by EDS analysis and electron diffraction pattern analysis.
When a stainless steel plate having such a precipitated phosphide is processed and strained, dislocation movement is hindered by the phosphides arranged in parallel to each other. As a result, it is considered that this phosphide exhibited the same effect as the crystal grain boundary and contributed to the suppression of rough processing.

The precipitation amount Pp of P is measured by the following electrolytic extraction residue method.
A test piece having a size of about 30 mm square is cut out from the center in the width direction of the stainless steel plate, and the entire surface of the test piece corresponding to the surface of the steel plate is wet-polished with water-resistant abrasive paper of number # 600. After polishing, the test piece base material (stainless steel base material) is dissolved by electrolysis at a constant potential of −100 mV in a methanol solution containing 10% maleic anhydride and 2% tetramethylammonium chloride. After the electrolysis, the residue (precipitate) remaining in the solution without being dissolved is captured using a 200 nm mesh filter. The trapped precipitate is washed with pure water and dried. Next, the precipitate is dissolved with aqua regia and perchloric acid, and elemental analysis is performed using ICP emission spectroscopy in accordance with JIS G 1258 to determine the mass of P in the precipitate. The amount of P obtained is divided by the amount of mass change of the test piece due to electrolysis (“the weight of the test piece before electrolysis” − “the weight of the test piece after electrolysis”) and expressed as a percentage. The amount of precipitation Pp ”(mass%).

(IV) Next, the manufacturing method of the ferritic stainless steel sheet of this embodiment is demonstrated.
The manufacturing method of the ferritic stainless steel sheet according to the present embodiment is a combination of hot rolling, hot-rolled sheet annealing, cold-rolling and cold-rolled sheet annealing (finish annealing). I will do it. That is, as an example of the manufacturing method, for example, a manufacturing method including steps of steelmaking, hot rolling, hot rolled sheet annealing, cold rolling, and cold rolled sheet annealing (finish annealing) can be employed.
In this embodiment, the conditions to be controlled in order to satisfy both the important crystal grain size and the precipitation state of the phosphide as described above are the conditions of heat treatment after hot rolling (hot-rolled sheet annealing), cold rolling. Rate, conditions for heat treatment after cold rolling (cold rolled sheet annealing), and other processes and conditions are not particularly limited.

After the hot rolling, heat treatment (hot-rolled sheet annealing) is performed at a temperature of 850 ° C. or more and 900 ° C. or less to ensure the precipitation amount Pp of the phosphide after the heat treatment. When the heat treatment temperature is less than 850 ° C., recrystallization failure occurs in the center portion of the plate thickness, and there is a possibility of causing deterioration in formability due to a decrease in r value and deterioration in polishing characteristics after processing due to ridging. For this reason, the minimum of the heat processing temperature of hot-rolled sheet annealing shall be 850 degreeC or more. Desirably, it is 860 degreeC or more. On the other hand, if the heat treatment temperature exceeds 900 ° C., the amount of phosphide precipitated is insufficient, and the above-described precipitation amount Pp cannot be ensured. Therefore, the upper limit of the heat treatment temperature for hot-rolled sheet annealing is set to 900 ° C. or less. Desirably, it is 880 degreeC or less, More preferably, it is less than 870 degreeC. In addition, since the precipitation state is hardly changed in the annealing after the cold rolling (finish annealing), it is important to control the precipitation amount Pp of P at this stage. It is preferable that the amount of P existing as a phosphide (P precipitation amount Pp) is 0.003% by mass or more by hot-rolled sheet annealing at a stage after the hot-rolled sheet annealing.

The rolling rate in the subsequent cold rolling is 75% or more and 90% or less.
In order to make the recrystallized grain size fine by the heat treatment performed after cold rolling, it is necessary to increase the amount of strain introduced. Recrystallization starts from the part where a lot of strain is introduced. In other words, the larger the amount of processing (the higher the rolling rate), the smaller the recrystallized grain size because there are more parts (nuclei) that are the starting points of recrystallization. For these reasons, in order to increase the crystal grain size number (decrease the crystal grain size), a higher rolling ratio is better. If the rolling rate is less than 75%, these effects cannot be obtained, and the r value may be lowered to deteriorate the formability. For this reason, in this embodiment, a rolling rate shall be 75% or more. Moreover, since r value improves, so that a rolling rate is high, it is desirable that a rolling rate is 80% or more. On the other hand, if the rolling rate exceeds 90%, the r value decreases, and the formability may decrease. Therefore, a rolling rate shall be 90% or less of range.

After the cold rolling, heat treatment (cold rolled sheet annealing) is subsequently performed, and this embodiment is characterized in that this heat treatment is performed rapidly. Specifically, in cold-rolled sheet annealing, the average rate of temperature increase in the temperature range of 400 ° C. to 800 ° C. is set to 80 ° C./s or more in the temperature increasing process. The maximum temperature reached is 880 ° C. or higher and 980 ° C. or lower. Cooling is started within 5 seconds after reaching the maximum temperature, and the average cooling rate in the temperature range from the maximum temperature to 700 ° C. is set to 50 ° C./s or more.
The “average rate of temperature increase in the temperature range of 400 ° C. to 800 ° C.” as used in the present embodiment refers to the time required for temperature increase in the temperature range of the steel plate temperature increase range (400 ° C.) in the temperature range. The value divided by. “Average cooling rate in the temperature range from the highest temperature to 700 ° C.” means the temperature drop width of the steel plate from the highest temperature to 700 ° C. from the time when the highest temperature was reached to 700 ° C. The value divided by the required time. Moreover, all the temperature (degreeC) in the following description points out steel plate temperature.

As described above, in the present embodiment, the phosphide precipitated by hot-rolled sheet annealing is crushed by cold rolling to form a precipitated state parallel to the cold rolling direction, and recrystallization is performed while maintaining this precipitated state. Go and get the product board. And even if the product plate provided with the phosphide in the above-described precipitation state is molded and subjected to distortion, the movement of dislocations can be hindered by the phosphide, and therefore it is possible to suppress rough processing.
For this reason, it is important to carry out the cold-rolled sheet annealing under conditions that allow recrystallization while maintaining the precipitation state after cold rolling.

In order to maintain the precipitation state after cold rolling and to obtain a rough working-resistant effect, the average heating rate in the temperature range of 400 ° C to 800 ° C in the temperature rising process is set to 80 ° C / s or more and the maximum temperature is reached. Start cooling within 5 seconds. That is, the temperature is rapidly increased in the temperature range of 400 ° C. to 800 ° C. at an average temperature increase rate of 80 ° C./s or more, and heated to the highest temperature reached (880 ° C. or higher and 980 ° C. or lower), and the holding time at the highest temperature reached. The cooling is started within 5 seconds. In this embodiment, when the temperature is held at the maximum temperature, the temperature may be kept constant, but within the range of the maximum temperature ± 10 ° C. (maximum temperature −10 ° C. to maximum temperature + 10 ° C.). If it is present, it is permissible even if the holding temperature varies. However, when the holding temperature fluctuates within the above range, it is necessary to control the temperature so as not to deviate from the appropriate range (880 ° C. or higher and 980 ° C. or lower) of the highest temperature reached.

When the average rate of temperature increase in the temperature range of 400 ° C. to 800 ° C. is less than 80 ° C./s or the holding time is longer than 5 seconds, the phosphide may be dissolved and the amount of precipitation as a product may not be ensured. In addition, rapid temperature increase in the temperature range of 400 ° C. to 800 ° C. has the effect of reducing the recrystallized grain size and is effective in suppressing rough processing. Further, when the temperature is rapidly raised in the presence of precipitates, the grain growth is suppressed by the pinning effect of the precipitates, so that there is an effect of further miniaturizing the product particle size and further suppressing roughness of the processed skin. From such a viewpoint, the average rate of temperature increase in the temperature range of 400 ° C. to 800 ° C. is preferably 150 ° C./s or more.
Further, from the viewpoint of maintaining the precipitation state of the phosphide, the holding time at the highest temperature is desirably 2 seconds or less. The holding time may be 0 seconds, that is, the cooling may be started as soon as the maximum temperature is reached.

In this embodiment, since the temperature raising process is performed by rapid heating, the time required for temperature raising is short. In order to complete recrystallization in this short time, the maximum temperature reached is 880 ° C. or higher. If the maximum temperature reached is less than 880 ° C., recrystallization becomes insufficient, and the workability may deteriorate due to a decrease in elongation. Therefore, in this embodiment, the maximum temperature reached is 880 ° C. or higher, preferably 900 ° C. or higher. On the other hand, when the crystal grain growth after the completion of recrystallization progresses, there is a risk that the roughening resistance to processing skin may be deteriorated due to the coarsening of crystal grains or insufficient precipitation due to phosphide solid solution. Is the upper limit. Desirably, it is 950 degrees C or less.

When crystal grain growth or phosphide solid solution progresses during the cooling process, the rough surface resistance to processing deteriorates, so the lower limit of the average cooling rate in the temperature range from the highest temperature to 700 ° C. is set to 50 ° C./s or more. Desirably, it is 100 ° C./s or more. The upper limit of the average cooling rate in the temperature range from the highest attained temperature to 700 ° C. is preferably 500 ° C./s or less.

In cold-rolled sheet annealing, it is possible to secure a phosphide and obtain a recrystallized structure by heat treatment for a long time in a lower temperature range than the above conditions, but the crystal grain size becomes large and the rough skin resistance characteristics Deteriorates. Furthermore, the effect of suppressing the roughening of the work-resistant skin is exhibited only when the precipitation state of the phosphide in the grains is in a state aligned in parallel with the rolling direction. For this reason, even if a phosphide is precipitated in the process of cold-rolled sheet annealing, it does not exhibit the effect. That is, it is important to perform cold-rolled sheet annealing under the above-described conditions that can control the precipitation state of the phosphide by cold rolling and maintain this precipitation state.

The ferritic stainless steel sheet according to the present embodiment can be manufactured by the manufacturing method described above.
In the present embodiment, the hot-rolled sheet annealing and the cold-rolled sheet annealing may be batch-type annealing or continuous-type annealing. Each annealing may be bright annealing performed in a non-oxidizing atmosphere such as hydrogen gas or nitrogen gas if necessary, or may be performed in the air.

Further, the thickness applied to the ferritic stainless steel plate of the present embodiment is not particularly limited, but is desirably 0.5 mm or more, preferably 0.6 mm or more from the viewpoint of ensuring strength. This is because when the plate thickness is thin, the strength of the molded part may be insufficient. It is necessary to design in consideration of the size and shape of parts to be manufactured, load resistance, and the like.

As described above, according to the present embodiment, it is possible to provide a ferritic stainless steel sheet that is excellent in formability and resistance to roughening after forming. Moreover, since the ferritic stainless steel sheet of the present embodiment is excellent in resistance to rough processing, it is particularly suitable for applications that require polishing to remove surface irregularities (skin roughness) after forming.

Next, examples of the present invention will be described. The conditions in this example are one example of conditions used to confirm the feasibility and effects of the present invention, and the present invention is not limited to the conditions used in the following examples. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the requirements of the present invention.
In addition, the underline in the table | surface shown below shows what has remove | deviated from the range of this embodiment.

Stainless steel having the composition shown in Table 1 was melted and cast into a slab, and the slab was rolled to a predetermined plate thickness by hot rolling. Thereafter, hot-rolled sheet annealing, cold rolling, and cold-rolled sheet annealing were performed to obtain a 0.6 mm thick stainless steel plate (product plate) No. 1-44 were produced. Heat treatment temperature (annealing temperature) of hot-rolled sheet annealing, cold rolling rate, average heating rate between 400 and 800 ° C in cold-rolled sheet annealing, maximum temperature reached, required time to start cooling (holding time), and maximum temperature reached The average cooling rate in the temperature range from temperature to 700 ° C. was changed as shown in Tables 2 to 4. The annealing time (holding time) in the hot-rolled sheet annealing was set in the range of 40 to 60 seconds.

Next, the obtained stainless steel plate No. 1-No. A test piece was cut out from the vicinity of the width center of 44, and the crystal grain size number (GSN) was measured by a line segment method in accordance with JIS G 0551 (2013). When measuring the crystal grain size, the number of crystal grains crossing per sample was set to 500 or more from an optical microscopic microstructure photograph of the cross section of the test piece.

Furthermore, stainless steel plate No. 1-No. A sample having a diameter of 110 mm was cut out from No. 44, and a cup molding test with a drawing ratio of 2.2 was performed using a hydraulic molding tester. It has been found that the drawing ratio has a great influence on rough skin after cup molding, but other molding conditions have no effect. Note that the cup molding test conditions carried out this time are as follows: punch diameter is 50 mm, punch shoulder R is 5 mm, die diameter is 52 mm, die shoulder R is 5 mm, wrinkle holding pressure is 1 ton, clearance is 1.67 t on one side (t is plate Thickness). Further, as a lubricant between the sample and the punch, a rust preventive oil “Dafney Oil Coat Z3 (registered trademark)” manufactured by Idemitsu Kosan Co., Ltd. was applied. Thereafter, in order to protect the surface of the steel sheet after forming, a lubricating sheet “Naflon Tape TOMBO9001 manufactured by NICHIAS Corporation” was attached.

About the sample which could be shape | molded by the drawing ratio 2.2, the surface roughness after cup shaping | molding was measured and the roughened processing skin was evaluated.
Here, when the degree of surface roughness and variation of each part of the sample (molded product) after cup molding were investigated, it was found that there was variation between the inside and outside of the vertical wall portion. The survey results will be described in detail.
The present inventors investigated the surface roughness of each part of the sample after cup molding. The roughness of the processed skin after cup molding is not simply proportional to the crystal grain size and strain, as is generally known, and the formation of irregularities on the surface of the molded product is suppressed by contact with the mold during molding. Therefore, it has been found that the surface roughness is reduced. In particular, the outer wall of the vertical wall of the molded product has a strong force to be pressed against the mold during molding, and there is a competition between the generation of irregularities during molding and the suppression of irregularities due to contact with the mold. It was found that the roughness of the scatter increases greatly at each measurement position. Therefore, it was considered inappropriate to evaluate the roughness of the processed skin after cup molding on the outer wall of the vertical wall.
Therefore, the surface roughness of the inner wall of the vertical wall portion with a relatively small force pressed against the mold was measured. As a result, it was found that the surface roughness after cup molding can be measured with high accuracy. Further, since the inner wall has a larger surface roughness than the outer wall, the inner wall having the larger roughness takes the most polishing time in the polishing step after molding. For this reason, it is considered appropriate to measure the roughness of the surface (evaluation of roughness of the processed skin) assuming polishing after molding on the inner wall of the vertical wall portion of the molded product. If the rough evaluation of the processed skin is good for the inner wall of the vertical wall portion of the molded product, it can be determined that the outer wall is also good.

The surface described in JIS B 0601 using a two-dimensional contact-type surface roughness measuring instrument for a length of 5 mm parallel to the height direction at the inner height central portion of the vertical wall portion of the sample after cup molding. Roughness measurement was performed, and arithmetic average roughness Ra was calculated. Based on the arithmetic average roughness Ra of 1.00 μm, when the Ra is less than 1.00 μm, the rough skin evaluation is judged to be good (“◯”), and when the Ra is 1.00 μm or more, the rough skin evaluation is poor (“ X ").

Further, in the same manner as described above, the precipitation amount Pp of P on the product plate was measured by the electrolytic extraction residue method.
First, a test piece having a size of about 30 mm square was cut out from the center in the width direction of the stainless steel plate, and the entire test piece corresponding to the surface of the steel plate was wet-polished with water-resistant abrasive paper of number # 600. After polishing, the test piece base material (stainless base material) was dissolved by electrolysis at a constant potential of −100 mV in a methanol solution containing 10% maleic anhydride and 2% tetramethylammonium chloride. After electrolysis, the residue (precipitate) remaining in the solution without being dissolved was captured using a 200 nm mesh filter. The trapped precipitate was washed with pure water and dried. Next, the precipitate was dissolved with aqua regia and perchloric acid, and elemental analysis was performed using ICP emission spectroscopy in accordance with JIS G 1258 to determine the mass of P in the precipitate. The amount of P obtained is divided by the amount of mass change of the test piece due to electrolysis (“the weight of the test piece before electrolysis” − “the weight of the test piece after electrolysis”) and expressed as a percentage. Precipitation amount Pp "(mass%).
In addition, it measured by the same method also about the precipitation amount Pp of P in the hot rolled annealing board before performing cold rolling.
The measurement results and evaluation results are shown in Tables 5 to 7.

As shown in Tables 2 to 7, according to this embodiment, by controlling the precipitation amount of the phosphide by optimizing the annealing conditions and rolling conditions, the surface roughness after processing was excellent and the moldability was excellent. It was found that a ferritic stainless steel sheet can be obtained.
In the example of the present invention, Ra <1.00 μm, and rough processing was suppressed.

On the other hand, Nos. 25 and 26 are examples in which the component composition was out of the range, but in both cases, the precipitation amount Pp and the crystal grain size number of P were within the range of the embodiment, but the formability deteriorated and could not be narrowed down. It was. No. 27 and 28 are examples using steel L without addition of Ti and Nb, but the immobilization of P is insufficient and the precipitation amount Pp of P becomes less than 0.001%, and the formability deteriorates. I couldn't squeeze it.
No. In Nos. 3 and 22, the average heating rate during cold-rolled sheet annealing was too low, so that the solid solution of the phosphide progressed and the precipitation amount Pp of P was insufficient. Furthermore, the crystal grain size number became smaller, and the roughened processing surface deteriorated.
No. In 5, 10, 12, and 24, since the holding time was too long, the solid solution of the phosphide progressed and the precipitation amount Pp of P was insufficient. Furthermore, the crystal grain size number was also reduced, and the roughened processing surface was deteriorated.
No. Nos. 6 and 15 had a low annealing temperature at the time of hot-rolled sheet annealing and an average temperature increase rate that was too low.
No. In No. 7, since the cold rolling rate was small and the maximum temperature reached was too high, the grain growth progressed, the crystal grain size number became small, and the roughness of the processed skin deteriorated.
No. In No. 9, since the annealing temperature at the time of hot-rolled sheet annealing was too high, the precipitation amount Pp of P could not be ensured, and the roughness of the processed skin deteriorated.
No. In No. 16, since the maximum temperature reached was too high, the crystal grain size number was reduced, and the roughened processing surface was deteriorated.
No. No. 19 had a low average heating rate during cold-rolled sheet annealing, and the holding time was too long, so that the solid solution of the phosphide progressed and the precipitation amount Pp of P was insufficient. Furthermore, the grain size number was also reduced, and the roughness of the processed skin was deteriorated.
No. No. 20 had a smaller crystal grain size number because the cold rolling rate was too small. As a result, the roughness of the processed skin deteriorated.
No. In No. 21, since the annealing temperature at the time of hot-rolled sheet annealing was too high, the precipitation amount Pp of P could not be ensured, and the roughened workability deteriorated.
No. In No. 14, since the maximum temperature reached was too high, grain growth progressed, the crystal grain size number became smaller, and the roughening of the processed skin deteriorated.
No. No. 31 had a low average cooling rate during cold-rolled sheet annealing, so that the solid solution of the phosphide progressed, the precipitation amount Pp of P became insufficient, the grain size number became smaller, and the roughened workability deteriorated.
No. No. 32 had a low average cooling rate during cold-rolled sheet annealing, so that the solid solution of the phosphide progressed and the precipitation amount Pp of P was insufficient, resulting in deterioration of the roughened work surface.
No. In No. 36, since the annealing temperature at the time of hot-rolled sheet annealing was too high, the precipitation amount Pp of P could not be ensured, and the processed skin roughness deteriorated.
No. No. 38 had a low average temperature increase rate during cold-rolled sheet annealing, and the maximum temperature reached was too high, so that the grain growth progressed and the grain size number became small, and the roughened workability deteriorated.

Further, in FIG. 2, in the region where the particle size number is 9.0 or more and the amount of precipitated P is less than 0.003%, the roughening of the processed skin can be expected to be somewhat reduced due to the relatively fine particles, but there is no effect of suppressing the roughened processing skin due to the P compound. For this reason, it is inferior in the rough surface resistance to processing compared with the present invention example having a similar particle size number and a large amount of precipitated P.

For steel components with P less than 0.003%, No. in Table 2 to Table 7. When manufactured in the same manner as in No. 4, the amount of precipitated P was 0.003% or less, and Ra after the molding test was 1.00 μm or more. For steel compositions with P over 0.1%, No. 2 in Tables 2-7. When manufactured in the same manner as in No. 4, the moldability was inferior and molding was impossible.

According to this embodiment, it is possible to provide a ferritic stainless steel sheet excellent in forming workability and resistance to rough processing after forming, and a method for manufacturing the same. For this reason, the ferritic stainless steel plate of this embodiment is applied suitably for a forming use.

Claims (4)

1. In mass%
   Cr: 11.0% or more and 30.0% or less,
   C: 0.001% to 0.030%,
   Si: 0.01% or more and 2.00% or less,
   Mn: 0.01% or more and 2.00% or less,
   P: 0.003% to 0.100%,
   S: 0.0100% or less,
   N: 0.030% or less,
   B: 0% or more and 0.0025% or less,
   Sn: 0% to 0.50%,
   Ni: 0% or more and 1.00% or less,
   Cu: 0% or more and 1.00% or less,
   Mo: 0% or more and 2.00% or less,
   W: 0% to 1.00%,
   Al: 0% or more and 1.00% or less,
   Co: 0% to 0.50%,
   V: 0% or more and 0.50% or less,
   Zr: 0% or more and 0.50% or less,
   Ca: 0% or more and 0.0050% or less,
   Mg: 0% or more and 0.0050% or less,
   Y: 0% or more and 0.10% or less,
   Hf: 0% or more and 0.10% or less,
   REM: 0% or more and 0.10% or less,
   Sb: 0% or more and 0.50% or less,
   Ti: 0.40% or less, Nb: 0.50% or less, including either one or both, the balance consists of Fe and impurities,
   The amount of P present as a phosphide is 0.003% by mass or more,
   A ferritic stainless steel sheet having a grain size number measured by JIS G 0551 of 9.0 or more.
2. In mass%,
   B: 0.0001% to 0.0025%,
   Sn: 0.005% or more and 0.50% or less,
   Ni: 0.05% or more and 1.00% or less,
   Cu: 0.05% or more and 1.00% or less,
   Mo: 0.05% or more and 2.00% or less,
   W: 0.05% or more and 1.00% or less,
   Al: 0.05% or more and 1.00% or less,
   Co: 0.05% or more and 0.50% or less,
   V: 0.05% or more and 0.50% or less,
   Zr: 0.05% or more and 0.50% or less,
   Ca: 0.0001% to 0.0050%,
   Mg: 0.0001% or more and 0.0050% or less,
   Y: 0.001% or more and 0.10% or less,
   Hf: 0.001% or more and 0.10% or less,
   REM: 0.001% or more and 0.10% or less,
   2. The ferritic stainless steel sheet according to claim 1, comprising one or more of Sb: 0.005% or more and 0.50% or less.
3. A hot rolling step of hot rolling the steel having the component according to claim 1 or 2,
   After the hot rolling step, a hot-rolled sheet annealing step of performing heat treatment at a temperature of 850 ° C. or higher and 900 ° C. or lower,
   After the hot-rolled sheet annealing step, a cold rolling step for rolling at a rolling rate of 75% or more and 90% or less,
   A cold-rolled sheet annealing step that follows the cold rolling step,
   In the cold-rolled sheet annealing step, the average rate of temperature increase in the temperature range of 400 ° C. to 800 ° C. in the temperature increasing process is 80 ° C./s or more, and the maximum temperature reached is 880 ° C. or more and 980 ° C. or less. The cooling is started within 5 seconds after reaching the maximum temperature, and is cooled at an average cooling rate in the temperature range from the maximum temperature to 700 ° C at 50 ° C / s or more. The manufacturing method of the ferritic stainless steel plate of description.
4. A hot rolling step of hot rolling the steel having the component according to claim 1 or 2,
   After the hot rolling step, heat treatment is performed at a temperature of 850 ° C. or higher and 900 ° C. or lower, and a hot rolled sheet annealing step in which the amount of P existing as a phosphide is 0.003% by mass or more,
   After the hot-rolled sheet annealing step, a cold rolling step for rolling at a rolling rate of 75% or more and 90% or less,
   A cold-rolled sheet annealing step that follows the cold rolling step,
   In the cold-rolled sheet annealing step, the average rate of temperature increase in the temperature range of 400 ° C. to 800 ° C. in the temperature increasing process is 80 ° C./s or more, and the maximum temperature reached is 880 ° C. or more and 980 ° C. or less. The cooling is started within 5 seconds after reaching the maximum temperature, and is cooled at an average cooling rate in the temperature range from the maximum temperature to 700 ° C at 50 ° C / s or more. The manufacturing method of the ferritic stainless steel plate of description.

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